The use of systems and methods to monitor the movements and locations of a variety of objects, including individuals, pets or even items of personal property, is well known. Every year, thousands of infants and young children are lost or abducted, particularly in venues such as shopping malls and amusement parks. As any law enforcement official will confirm, the first few minutes after the child is determined to be missing are the most critical in locating the child. Pets and even items of personal property are also lost or stolen on a regular basis. Here again, the initial moments after the discovery of the loss or theft of the pet or object are the most critical in any successful effort to locate them. In the past, telemetry and radar have been used to effectively track and locate objects, both on the ground, in the water and in outer space. This type of tracking utilizes the emission of a signal from a source and the reflection of the same signal received at the source from the target being tracked. Other means of tracking objects are also known. For example, the transmitter of U.S. Pat. No. 5,119,072 includes an antenna that radiates a frequency modulated rf carrier signal to a receiver. The receiver contains circuitry for monitoring field strength of the carrier signal and for indicating range from the receiver to the transmitter as a function of such field strength. Another example is U.S. Pat. No. 5,021,794, which discloses a locator system comprising a radio transmitter concealed on the person to be located which activates and transmits a coded UHF radio homing signal upon the receipt of an initiating signal containing the person's specific code. Tracking vehicles with automatic UHF radio direction finding and distance measuring equipment are used for locating the source of the homing signal. Other examples are U.S. Pat. No. 4,990,892, which discloses a personnel locator system using infrared transmitters and receivers to monitor classes of individuals; U.S. Pat. No. 4,868,544, which discloses a shopping cart retrieval system comprising a VHF beacon radio transmitter that continuously emits a signal and a VHF receiver with an omnidirectional and directional antenna; and, U.S. Pat. No. 4,899,135, which discloses a child monitoring device that includes two or more transceivers operating in the radio or ultra sonic frequency ranges.
Monitoring systems and methods of the types generally referred to hereinabove may include a transmitter or monitoring unit, and a portable receiver removably attached to the person or object being monitored to receive a signal broadcast by the transmitter. The most common of these systems or methods employs technology that is capable only of determining the precise distance the person or object is from the transmitter or monitoring unit, and possibly may include means to alert the transmitter, by sounding an alarm, when the person or object exceeds a predetermined distance from the monitoring unit. The large majority of these systems include no capability for determining the direction of the person or object being monitored, and in these few instances where this capability exists, it does so at a tremendously high price due, in large part, to the involvement of complex technology and equally high manufacturing costs. In contrast to the prior art systems and methods, the present invention employs technology that combines the capability of determining accurately both the distance and the direction of an individual or object being monitored relative to the transmission or monitoring point, and achieves this at a relatively low cost of manufacturing.